Constant tension stretch wrapping machine

ABSTRACT

A wrapping machine and operation for wrapping a load arranged on a pallet with a stretchable wrapping material under a substantially constant tension during the wrapping operation. The load that is to be wrapped is arranged on a support member which can be rotated for drawing off stretchable wrapping material from a roll of such material. A dispensing mechanism holds the roll of stretchable wrapping material and dispenses the material so that it can be wrapped around the load arranged on the support member. As an alternative to the support member with the load being rotated, the dispensing member can revolve around the support member and the load so as to dispense and wrap the load with the stretchable material. During the wrapping operation, the tension on the portion of the stretchable wrapping material being wrapped around the load is maintained at a substantially constant tension. Prior to actually wrapping the load with the stretchable wrapping material, such material can be prestretched. The prestretching operation occurs at a location between the dispensing mechanism and the location at which the stretchable material is actually supplied to the load for wrapping.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 839,531, filed Mar.14, 1986, now U.S. Pat. No. 4,706,433, which is a continuation of U.S.Ser. No. 374,741, filed May 4, 1982, now U.S. Pat. No. 4,590,746, whichis a continuation-in-part of U.S. Ser. No. 307,283, filed Sept. 30,1981, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to equipment and a process for wrapping apallet load with a stretchable wrapping material in such a manner so asto achieve the tightest wrap possible without damaging the load beingwrapped.

2. Description of the Prior Art

United Kingdom Pat. No. 2,059,906 discloses a stretch wrapping machineand process. An accumulator having a dancer roller which contacts themoving film, compensates for variations in speed at which the film isdrawn into the object being wrapped. The dancer roller is biased againstthe moving film by a constant force producing element.

Several different types of stretch wrapping machines are illustrated inU.S. Pat. Nos. 4,050,221; 4,077,179; and 4,079,565, all to Lancaster etal. Other stretch wrapping machines are shown in U.S. patentapplications Ser. Nos. 72,471, filed Sept. 4, 1979 now U.S. Pat. No.4,299,076 to Humphrey, and 235,946, filed Feb. 19, 1981 now U.S. Pat.No. 4,628,667 to Humphrey et al, both of which are assigned to the sameassignee as the present application. The subject matter of both of theseapplications is hereby incorporated by reference.

Stretch wrapping a pallet lead may be compared to stretching a rubberband around a group of objects. Presuming that there is auniformly-shaped pallet load that has been stretch wrapped with astretchable wrapping film tensioned to a 10 pound pull and that thereare three wraps, i.e. the load has been wrapped three times around theload, then the force holding the load together are 30 pounds in bothdirections at each corner. At the center points of each side of the loadthere is no direct inward holding force although the products in thecenter of the load are clamped together by the adjacent outer products.There is also a diagonal force which is the resultant of the twodirectional forces on each corner of the load.

It must be recognized that all wrapping films relax with the result thatthe rubber band effect or holding power is diminished with the passingof time. The amount of tension required for a particular load must besufficient to contain the integrity of the load which may settle, changeshape or shift in transit or during storage. A shifting load alternatelystretches and relaxes the film; each time the film is stretched further,the recovery tension is reduced. Enough initial tension must be appliedto the load to compensate for these subsequent.

If the extent to which the wrapping material is stretched is too great,then this will diminish its holding power. Most films produce theirgreatest holding force or tension when stetched during the wrappingoperation between approximately 20% to 35%. If a film is stretchedbeyond its elastic limit, which is the point where permanent deformationoccurs, the film thins out in gauge and its ability to recover and holdthe load will decrease, perhaps drastically and perhaps even destroyed.For example, a wrapping film with a maximum power of 30 pounds whenstretched to 30% stretch may have only 15 to 17 pounds holding power ifstretched over 100%. If the resulting weaker film is adequate to holdthe load securely then economies will result by using less film perpallet load. Otherwise, additional wraps will be required to obtain thesame holding force as can be obtained by fewer of the same filmstretched only 20% to 35%.

As indicated above, all stretch films relax to some extent, in varyingdegree, after the load is wrapped. In addition, the wrapping films whichhave been stretched or prestretched more than 100% will tend to relaxmore than films which are stretched 20% to 35% during the wrappingprocess. The objective in wrapping a load with a stretchable wrappingmaterial is to obtain the tightest wrap possible without damaging orcausing collapse of the load.

The concept of prestretching the stretchable wrapping material beforewrapping a load has recently emerged and has been incorporated inseveral commercial machines. Employing a film prestretching device makesthe film longer and thinner while simultaneously increasing the yieldand decreasing the load holding power or strength of the film. In orderto stretch the stretchable wrapping film being wrapped around the load,the load is rotated at a speed for drawing up film faster than the filmis dispensed. In the preferred embodiment of the present invention, thefilm can be pre-stretched up to 300% and then further stretched duringthe wrapping operation to cause a total stretching of 500%. Sincevarious loads, however, may need to be wrapped with film havingdifferent holding powers, prestretching enables a single type of film tobe used in wrapping a plurality of different types of loads with thefilm then being prestretched to the extent necessary.

Whether or not a prestretching device is used, however, the film muststill be stretched by applying tension on the film as it is applied tothe load. This tension, which results from the load pulling the filmagainst some restraining device during the wrapping operation, isnormally necessary since it causes the stretching that provides theholding power to hold the load together. Prestretching is a separate andisolated function from stretch wrapping of the load. Whether filmprestretching is done on the wrapping machine or in the filmmanufacturers plant, the film normally must still be further stretchedduring the wrapping operation.

The currently available commercial prestretching devices normallyconsist of two rubber-covered rollers which are rotated at differentspeeds. The speed differential is created by appropriate gears, beltdrives, separate D.C. variable speed motors, separate brakes or othersimilar types of mechanisms. While some of the prestretching devices arepowered by the load pulling the film, most of the devices are motorpowered.

In the operation of the pre-stretching devices, the film passes overboth rollers with the second roller rotating at a faster speed than thefirst roller thereby providing a stretching action on the film. If thesecond roller rotates twice as fast as the first, assuming there is noslippage and a minimum "necking down" of the film, the film will bestretched approximately 100%. Various speed ratios of the rollers willproduce proportional percentage stretch in the film. For example, arelatively heavy gauge film (#90) may theoretically be doubled in yield(length) to wrap light loads with light tension. The original holdingpower of such film was 30 pounds but with 100% pre-stretching the gaugechanges to #45 and the holding power is approximately 16 pounds, whichis adequate for light loads.

Loads consisting of cartons which are easily crushed may be stretchwrapped with light gauge film under very low tension. Light gauge filmsare on the market in the range of 60 to 70 gauge but these films costmore per pound. The pre-stretching devices permit the use of lower costheavy gauge films which may be pre-stretched to make light gauge filmswhich are then wrapped around the load under light tension. For example,a 5000 foot roll of 100 gauge film, if pre-stretched to 100% would yield10,000 feet of 50 film and wrap twice as many pallet loads which may beadequately wrapped under light tension.

There are several different types of pre-stretching systems. A firsttype is a non-powered system which relies upon the load pulling the filmboth off the roll of film and through the pre-stretching device. Thissystem eliminates the requirement for friction brakes but considerableinertia is added to the operation of the system both due to thenecessity of pulling the material off of the film roll and due to theneed to rotate the rubber rollers which are geared together. Inaddition, the gears between the rollers must be changed in order tochange the percentage of pre-stretching.

A second type of system is a powered system that employs a variablespeed motor drive for each of the two rubber rollers plus a frictionbrake on the film roll. The two motors must attempt to maintain a speeddifferential under varying film demands which often leads to severeinaccuracies in the extent of pre-stretching. In addition, the frictionbrake adds inherent erratic behavior toward trying to maintain aconstant pre-stretching.

A second type of powered system uses one power driven roll with afriction brake then being coupled to the other roller. Thepre-stretching is adjustable by varying the setting of theelectromagnetic brake torque of the friction brake.

In all of the above systems, however, there is still the need foradditional stretching of the film to take place between thepre-stretching device and the load when actually wrapping the load. Thisadditional stretching during the wrapping operation quite often variesin magnitude due to the uneven shape of the load, which is normally notround. The final result is that each load is still wrapped under adifferent stretch tension as well be further explained below.

While there are several benefits of pre-stretching, such as outlinedabove, there are also definite limitations to such procedures. In moststretch wrapping applications, the integrity of the unitized loaddepends upon the cling of one layer of film to another layer of film. Inaddition to keeping the tail end of the wrap from unwrapping, film clingis extremely important in spiral wrapping where the "lamination" oroverlaying of one layer upon another produces considerable strength inthe wrapped load. Many of the stretchable films currently on the markettend to dramatically lose their clinging ability when stretched beyond100%. Often loads that are stretch wrapped with 120% pre-stretched filmhave been observed to become unwrapped in 48 hours. Other films willbecome too brittle and shatter like glass when excessively pre-stretchedwhile yet other films will take a set and lose their load holding power.All films tested have demonstrated a considerable loss in strength inthe transverse direction when stretched over 100%.

This inherent limitation in the extent to which it is beneficial topre-stretch a film prior to wrapping becomes even more critical uponrealizing that further variable stretching occurs during the wrappingoperation itself. The rotating turntable support member rotates a loadwhich is usually not round. This results in variable demands on the filmdispensing mechanism. This becomes a particular problem where thedispensing of stretchable material does not occur at an identical ratewith the demand for such material by the rotating load. This isparticularly a problem where a pre-stretching device is inserted betweenthe film roll and the load since the roll of film does not roll freelyin response to the taking up of film by the load. Inherently thisvariable demand for film that is not synchronized with the dispensingmechanism produces an erratic tensioning of the film.

As can be seen from FIG. 8 of the drawings, which illustrates a priorart system, with a rectangular load the demand for film will varygreatly as the load is rotated. Assuming that the pallet load size is 40inches by 60 inches when the film is wrapping the long side of the load,the effective wrapping diameter is 40 inches. At 10 rpm turntable speed,the film is drawn from the roll at 105 fpm. As the corner of the loadapproaches, the effective wrapping diameter becomes the diagonaldimension of 72 inches. At 10 rpm turntable speed, the film must nowaccelerate to 188 fpm in less than 1 second, i.e. an 80% increase inspeed. Subsequent rotation of the load produces a similar decrease inspeed and a series of sudden accelerations and decelerations as the loadcontinues to be wrapped. The film speed curve abruptly changes duringthe wrapping operation. The inertia and momentum of the film rollconstantly opposes the desired results by tightening and slackening filmtension during the wrapping cycle which further enhances the degree ofvariance in tension of the wrapping film. In such a wrapping operation,the film tension at the corners of the load can double with the filmbrake turned off due entirely to the inertia of the film roll. The peakin film tension occurs just as the sharp corner of the load is presentedto the film. Consequently, the tension on the film must be set wellbelow (approximately half) the theoretical limits of the film to preventbreaking of the film.

This problem is further aggravated by the braking systems in common usetoday. Most of the braking systems employ some variation of a frictionbrake, usually electro-magnetically controlled. An electromagnetic brakehas approximately 300 to 400% more torque at rest (static) than whenrotating. This means in connection with the operation of the stretchwrapper that when the film tension drops off suddenly during loadrotation, the film roll stops turning momentarily and the brake becomestightly locked. As the corner of the load swings outwardly (see FIG. 8)the film tension suddenly increases and the friction brake must bejerked into rotation to reduce its braking torque to preset value. Thisviolent action occurs just as a relatively sharp corner of the loadcomes around and it is one of the major causes of film breakage. Thisproblem becomes even more pronounced with higher speeds of rotation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved stretchwrapping machine and stretch wrapping operation.

Another object of the present invention is to provide a stretch wrappingmachine for wrapping a load with a stretchable material whilemaintaining such material under a substantially constant tension.

A further object of a first embodiment of the present invention is toprovide a stretch wrapping machine that includes a pre-stretching deviceand is capable of wrapping a load with a stretchable wrapping materialwhile maintaining such material under a substantially constant tensionduring the wrapping operation.

Still another object of the present invention is to provide an improvedstretch wrapping machine for enabling the wrapping of a load with astretchable wrapping material and reducing the possibility of breakageof the material during the wrapping operation due to variations intension on the stretchable material.

A still further object of the present invention is to provide animproved stretch wrapping machine for more tightly wrapping loadswithout crushing any portion of the load.

Still another object of the present invention is to provide an improvedstretch wrapping machine capable of tightly wrapping irregularly shapedloads while maintaining a substantially constant stretch of thestretchable material.

A still further object of the present invention is to provide a stretchwrapping machine and a wrapping operation that enables the stretchablematerial to be pre-stretched to a greater extent without subsequentlyincurring a risk of film breakage during the wrapping operation.

A still further object of the present invention is to provide a stretchwrapping machine and operation that overcome those problems of priormachines discussed above.

Still another object of the first embodiment of the present invention isto provide a stretch wrapping machine that eliminates the need to usetroublesome friction brakes and provides a film dispensing system thatsupplies the stretchable film to the load during the wrapping operationunder a substantially constant tension and compensates for speed surgesresulting from uneven configuration of the load.

Another object of a second embodiment of the invention is to provide astretch wrapping machine having a friction brake which controls thesupply of film while maintaining a relatively constant tension on thefilm.

The above objectives are accomplished in the utilization of the stretchwrapping machine and stretch wrapping operation of the present inventionwherein the stretchable wrapping material is wrapped around the palletload while maintaining a substantially constant tension on the wrappingmaterial. The constant tension on the wrapping material is maintained byproviding a mechanism that in effect isolates the wrapping of the loadand the accompanying drawing up of material by the load from thedispensing of the stretchable material from the roll of material. Inthis manner the tension on the stretchable material can be maintainedsubstantially constant even though the rate at which material is takenup by the load varies due to the varying shape of the load.

The pallet load that is to be wrapped by the wrapping machine is placedupon a load support member. The support member then is normally rotatedso as to take up the stretchable wrapping material to be wrapped aroundthe load. Alternatively, the dispensing mechanism which holds thestretchable wrapping material can revolve around the load support memberso that the load pulls off the stretchable wrapping material as thedispensing mechanism revolves. A drive mechanism provides the relativemovement between the dispensing mechanism and the load support member.

A tension maintaining mechanism forming part of the wrapping machineacts to maintain the substantially constant tension on the stretchablematerial as the material is wrapped around the load. This tensionmaintaining mechanism applies a biasing force against a tension rollerover which the stretchable material passes so as to maintain a constanttension on the stretchable material. The tension roller is movable withrespect to the load support member. Thus as the load is rotated, if dueto the configuration the load draws up the stretchable material morerapidly then the tension roller moves closer to the load support memberso as to maintain the constant tension. Alternatively, as the speed atwhich the load draws up the stretchable material decreases, the tensionroller moves away from the support member so as to pick up any slackthat would otherwise occur in the stretchable material and therebymaintain the constant tension in such material.

The tension roller of the tension maintaining mechanism is attached to apair of arms capable of swinging either towards or away from the supportmember so in essence the tension roller with the arms act as a "dancer".To understand the operation, returning to prior art FIG. 8, it can beseen that as the corner of the load swings outwardly it suddenly demandsmore film. In accordance with the present invention to compensate forsuch demand, the dancer of the tension containing mechanism swingsforward so as to dispense surplus film stored in the dancer loop. As theshort side of the load comes around thus requiring less film than thedispensing mechanism in supplying, the dancer swings away so as to storethe surplus film. The biasing force on the dancer which is constantthroughout arcuate swing maintains the same tension on the film in anyposition of the dancer.

The dispensing mechanism includes a film feed roll for drawing film offthe roll of stretchable material at a constant rate thus the film feedroller acts as a governor that simply feeds the stretchable material ata constant rate. In both embodiments, a control device can beincorporated and operated in conjunction with the movement of the dancerso as to sense when the dancer moves too far in either direction. In thefirst embodiment when such movement of the dancer occurs, the speed ofthe film feed roller is adjusted by the control device to bring thesystem back into balance. In the second embodiment when such excessmovement of the dancer occurs the braking force which is applied to thefilm feed roll is varied to bring the system back into balance.

In order to maintain the biasing force on the dancer and hence thetension roller that presses against the stretchable material, a constantforce applying mechanism is coupled to the dancer to maintain theconstant force as the dancer swings back and forth. Two possible typesof mechanisms that can be used for supplying the necessary constantforce to the dancer are a fluid cylinder having a self-relieving typeair regulator and a Neg ator spring that is marketed by Hunter SpringCompany. Both of these mechanisms provide a constant force regardless ofdisplacement. A constant torque electric motor can also be used toproduce constant force. If the constant force mechanism biases (orloads) the tension roller with force of 100 lbs. then the tension forceon the stretchable material is 50 lbs. on the material extending fromeach side of the roller. The actual force applied by the fluid cylinderor spring, however would be higher since the length relationship of thelinkage members between the tension roller and the force applyingmechanism must be taken into consideration.

In the first embodiment, prior to actually wrapping the stretchablematerial around the load, it is possible to pre-stretch the material. Inorder to carry out such a pre-stretching operation the material ispassed between a first stretching roller and a second stretching rollerarranged along the path of travel of the material with the second rollerrotating with a greater circumferential speed than the circumferentialspeed of the first stretching roller. This differential in speed createsa corresponding pre-stretching of the stretchable material. The secondstretching roller of the pre-stretching mechanism can be the constantrate feed roller of the dispensing mechanism. Thus the only additionalroller is a first stretching roller the speed of which is varieddepending upon the extent of pre-stretching desired. In order to ensurethat the pre-stretching occurs to the extent desired as determined bythe speed differential between the two rollers, spring biased niprollers can be used to clamp the stretchable material against thestretching rollers to avoid any slippage of the material during thepre-stretching operation.

In conjunction with the utilization of the constant tension mechanism ofthe present invention, it is possible to employ a horizontal supplymechanism that constitutes another aspect of the invention. Thehorizontal supply mechanism enables a much larger roll of stretchablewrapping material to be used in conjunction with a wrapping machineparticularly a spiral type wrapping machine since the roll does not moveup and down with the spiral dispensing carriage. With this type ofwrapping machine, the film roll and the feed roller as well as thestretch roller are arranged horizontally in fixed positions so that eachrotates about a fixed horizontal axis. The film as it is dispensed fromthe roll passes over a 45° air bar that turns the film into a verticalposition just prior to being supplied to the dancer mechanism with thetension roller. Only the dancer mechanism with the tension roller andthe air bar move up and down so as to supply the film to the load inspiral application. This particular mechanism provides two principalbenefits. First, the mechanism eliminates the need to move the heavyweight of the film roll, stretch rollers, drive motors, controls andother associated equipment up and down on the vertical cage since all ofthese remain stationary. Thus a lighter, safer and more economicalelevator mechanism can be used. Second, it now becomes practical to usemuch larger film rolls which more easily can be mounted horizontally formost packaging machines particularly on automatic machines therebyenabling, for example, up to four times as many pallet loads to bewrapped without stopping the machine or changing the rolls.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a stretch wrappingmachine in accordance with the present invention.

FIG. 2 is a diagrammatic view illustrating the operation of the stretchwrapping machine of FIG. 1.

FIG. 3 is another diagrammatic view similar to FIG. 2 illustrating thestretch wrapping machine during another stage of the wrapping operation.

FIG. 4 is a diagrammatic view of the stretch wrapping machineillustrated in FIG. 1.

FIG. 5 is a top plan view of the dispensing mechanism of the stretchwrapping machine illustrated in FIG. 1.

FIG. 6 is a front plan view of the dancer mechanism and tension rollerused in the stretch wrapping machine illustrated in FIG. 1.

FIG. 7 is a perspective diagrammatic view of another embodiment of thestretch wrapping machine of the present invention.

FIG. 8 is a diagrammatic view of a stretch wrapping machine inaccordance with the prior art.

FIG. 9 is a top view of the preferred form of the first embodiment whichincludes a motor control mechanism for the stretch rollers.

FIG. 10 is an elevational view of a modification of the embodiment ofFIG. 9.

FIG. 11 is a view of a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A stretch wrapping machine 2 such as illustrated in FIG. 1 has arotatable load support turntable 4 and a dispensing mechanism 8 botharranged on a support frame 6. Turntable 4 is capable of rotating arounda central axis so that it draws off stretchable wrapping material (oftenreferred to as film), from the dispensing mechanism with the supply roll16.

The particular stretch wrapping machine illustrated in FIG. 1 is aspiral type wrapping machine in which the width of the wrapping materialis less than the height of the load that is to be wrapped. In theoperation of the spiral wrapping machine during the wrapping operation,dispensing mechanism 8 moves up and down carriage 10 as turntable 4rotates the load to be wrapped. In this manner the load is wrapped witha plurality of overlapping wraps as the dispensing mechanism moves upand down on the carriage. This movement of the dispensing mechanism iscontrolled by movement of drive chain 12 which can be driven in eitherdirection for moving dispensing mechanism 8 which is coupled to thedrive chain.

Dispensing mechanism 8 includes a supply roll of the stretchablewrapping material 16 which first passes around a first roller 18 andsubsequently over a feed roller 32 before being supplied to the constanttension mechanism. A motor drive 20 drives feed roller 32 which in turndrives roller 18. Alternatively motor drive 20 can be directly coupledto roller 18 and then feed roller 32 would be coupled to and driven byroller 18.

The output gear (sprocket) 23 of motor drive 20 is coupled to gear(sprocket) 25 and transfer drive shaft 26 through a drive chain 22.Transfer shaft 26 in turn rotates pulley 28 which through drive belt 24rotates pulley 30. By properly sizing the portions over which drive belt24 passes around pulleys 28 and 30, a desired rotating relationshipbetween the two pulleys and hence rollers 18 and 32 can be obtained. Avariable speed drive mechanism can be used between shaft 26 and shaft31. Rotation of pulley 30 through shaft 31 rotates roller 18 at a speedhaving a desired relationship to the speed of roller 32.

The speed at which feed roller 32 rotates should remain at a relativelyconstant set level for supplying the stretchable material to the tensionmechanism at a constant rate during a wrapping operation. The speed offeed roller 32 can vary under certain circumstances during a wrappingoperation, however, the speed of rotation of feed roller 32 can bevaried by varying the output speed of motor drive 20. This speed ofrotation is controlled through control mechanism 14, which is alsocapable of controlling other various aspects of the operation of thestretch wrapper such as the number of wraps to be made around the loadand the height to which the dispensing mechanism should travel inwrapping the load.

Gear 25 and transfer shaft 26 act as the coupling for driving roller 18and also drives feed roller 32. Due to the difference in sizes ofpulleys 28 and 30, as shown in FIG. 1, as well as in FIG. 5, feed roller32 will rotate at a much faster speed than roller 18. The ratio of thespeed of rotation of the circumferential surface of feed roller 32 ascompared to roller 18 determines the extent of pre-stretching that willoccur. If there is no slippage between the wrapping material and rollers18 and 32, then the extent of pre-stretching is directly proportional tothe speed differential between the two rollers. In order to prevent anyslippage, both roller 18 and feed roller 32 are covered with a rubbermaterial for firmly grasping the stretchable material as it passesaround the roller. In addition, nip rollers 34 and 36 which are springbiased by mechanism 56 as shown in FIGS. 4 and 5 press the nip rollersfor clamping the stretchable material against rollers 18 and 32 as therollers rotate so as to avoid any slippage of the stretchable material.

The operation of the constant tension mechanism can be seen and isexplained herein in conjunction with the diagrammatical illustrations inFIGS. 2 and 3 as well as comparison with the prior art system asillustrated in FIG. 8. As shown in FIG. 8, as load 78 rotates the speedat which the load rotates will vary. This can be seen by observing thedifference in the length of the stretchable material between roller 84and load 78 between the load positioned as shown by the solid lines andthe load positioned as shown by the broken lines. If roller 80 is powerdriven or rollers 82 and 84 are power driven so that the quantity ofmaterial passing around roller 84 is relatively constant during thewrapping operation then the variance in the demand for film by rotatingload 78 will increase the tension on the film thereby causingsignificant variations in the amount of stretching of the film duringthe wrapping operation. Even if the machine as shown in FIG. 8 suppliesthe film completely based upon the pulling of the material by rotatingload 78, due to the inertia in pulling the various roller the tension onthe wrapping material still will vary.

In the wrapping machine and operation of the present invention, however,this variance in the demand for wrapping material by the rotating load44 is compensated for by the constant tension mechanism so as tomaintain a constant tension on the section of stretchable wrappingmaterial 42a and 42b being supplied to and wrapped around load 44 as itrotates. As shown in FIG. 2, as load 44 rotates to a position where thespeed at which the load draws up the stretchable wrapping materialincreases, tension roller 38 moves closer to the load by pivotingmovement of dancer arms 40. As tension roller 38 moves closer, itsupplies the extra material needed while the tension force that pullsupon the portion of wrapping material 42a extending to the load ismaintained at a substantially constant level.

Dancer arms 40 on which roller 38 is mounted are connected to an aircylinder 52 having a self-relieving type air regulator, and through alinkage arm 50 and piston rod 54. The linkage arm 50 is connected to arm40. There can be either a single air cylinder or two cylinders, one atthe top and one at the bottom. With air cylinder 52, a constant biasingforce is applied to tension roller 38 for biasing such roller away fromload 44 with a substantially constant force irrespective of the positionof tension roller 38 and the extent to which piston rod 54 extends fromair cylinder 52. Thus whether the load 44 is in the position shown inFIG. 2 or the position shown in FIG. 3 the tension on the portion ofwrapping material extending to the load, 42a and 42b, respectively, ismaintained at a substantially constant level. In this manner, theadditional stretching of the stretchable material that takes placeduring the actual wrapping of the load can be maintained at asubstantially constant percentage. While the tension is maintainedconstant during a wrapping operation, the tension can be varied from oneoperation to the next by regulating or adjusting the air pressure to theair regulator of the air cylinder itself or the air cylinder positionwith respect to tension roller 38.

The stretching during the wrapping operation is separate from thepre-stretching that occurs prior to the material passing around tensionroller 38. As described above, pre-stretching of the stretchablematerial occurs between roller 18, which acts as a first stretchingroller, and feed roller 32, which acts as the second stretch roller. Tohelp avoid breakage in and necking down of the stretchable materialduring the pre-stretch operation, the stretchable material also can passover an idler roller 48. Idler roller 46 improves the wrap around roller18.

During the wrapping operation it is desirable to keep the geometricalangle of the film as it approaches tension roller 38 and as it leavesthe tension roller substantially the same. Variations in such angleshave been noted to have a negative impact upon the tension in the filmthereby causing undesirable variations in the stretching of the filmduring the wrapping operation. For this purpose, an additional idlerroller 58 can be inserted along the path of the stretchable wrappingmaterial as it leaves tension roller 38 such as shown in FIG. 4. Roller58 rotates about a fixed axis and hence the angle of the filmapproaching tension roller 38 and the film leaving tension roller 38remains substantially the same.

Tension roller 38 is connected to a pair of dancer arms 40 at both endsof the roller such as shown in FIG. 6. The two arms are then connectedto rod 60 which in turn is coupled to self-relieving air cylinder 52through linkage member 50 and piston 54.

In a modified embodiment of the stretch wrapper of the presentinvention, such as shown in FIG. 7, a role of stretchable wrappingmaterial 62 is arranged along a horizontal axis. The stretchablematerial as it leaves roll 62 passes over a feed roller 66 which isdriven in the same manner as feed roller 32 as discussed above inconnection with the embodiment of FIG. 1. The stretchable material alsocan pass over an intermediate roller 64 that serves as a stretch rollerby being at a slower speed than roller 66 for pre-stretching thestretchable wrapping material. By rotating roller 66 at a greatercircumferential speed than roller 64, pre-stretching of the stretchablewrapping material can be obtained. The stretchable wrapping materialthen passes around an idler 68 still traveling along a vertical path andsubsequently is rotated by 90° as it passes around air bar 70. Air bar70 has a plurality of openings out of which air passes so as to enable asmooth flow of the stretchable material around the air bar as thematerial is turned. Air bar 70 is arranged so as to extend at a 45°angle with respect to idler roller 68 so that it can properly rotate thestretchable wrapping material. The stretchable wrapping material thenpasses around tension roller 72 that is connected to dancer arms 74. Theportion of the stretchable wrapping material 76 passing around tensionroller 72 is then fed to the load for being wrapped around the load asdiscussed above in conjunction with the embodiments shown in FIG. 1.During a spiral wrapping operation the only elements that would move upand down along carriage 10 in the embodiments shown in FIG. 7 would beair bar 70 and the tension maintaining mechanism that includes tensionroller 72 and dancer arms 74.

FIG. 9 illustrates a form of the first embodiment of the invention whichcontrols the speed of the rollers 18 and 32 (not the relative speedbetween the rollers) in response to movement of the dancer arms 40. InFIG. 9, the angles "a", "b" and "c" are substantially equal to eachother within the path of rotation defined by the arcuate segments labled"INCREASES MOTOR SPEED, NO SPEED CORRECTION, and SLOWS MOTOR SPEED".When these angles are substantially equal, the wrapping tension ismaintained substantially constant. The roller 32 of the prestretcher andthe tension roller 38 define a path of approach of the stretchablewrapping material and the tension roller 38 and the roller 58 define apath of departure of the stretchable wrapping material. The linkage arm50 is connected to arm 40. If has been found that the control of thespeed of the rollers 18 and 32 in response to movement of the dancerarms 40 past angular limits 70 and 72 to increase the speed of therollers in response to an increase in the velocity of the wrappingmaterial at the load support and to decrease the speed of the rollers inresponse to a decrease in the velocity of the wrapping material at theload support helps prevent breakage of the wrapping material. Identicalreference numerals are used in FIGS. 1-8 and 9 to identify like parts.In addition to the constant tension mechanism described supra withregard to FIGS. 2 and 3, the form of the first embodiment illustrated inFIG. 9 includes means for varying the velocity of the first roller 18and feed roller 32 when the dancer arms 40 rotate past angular limits of70 and 72. The means for varying the velocity of the rollers 18 and 32includes a cam 74 which is fixedly mounted to the end of dancer arms40°. The rotation of the cam 74, which is produced by rotation of dancerarms 40, closes microswitches 80, 82, 84 and 86 as described, infra. Thecontact of microswitch 80 is closed when the dancer arms 40 rotatecounter-clockwise past the angular limit 70 and the contact ofmicroswitch 82 is closed if and when the film breaks. The closure of thecontact of microswitch 80 activates a motor control mechanism 88described infra to slow down the rotation of rollers 18 and 32 to reducethe supply of film to ultimately cause the dancer arms 40 to rotate backclockwise past angular limit 70 where no motor speed control isutilized. If the film breaks, the contact of microswitch 82 closes andcauses activation of an emergency stop dynamic braking circuit whichstops the motor instantaneously which drives the rollers 18 and 32. Thecontact of microswitch 84 is closed first when the dancer arms 40 rotateclockwise past angular limit 72 and the contact of microswitch 86 isclosed manually when the dancer arms 40 are pushed against theresistance of a spring (not illustrated) to run the rollers 18 and 32 ata slow speed used only during threading of the film. The closure of thecontact of microswitch 84 activates the motor control mechanism 88described infra to speed up rollers 18 and 32 to increase the supply offilm to cause dancer arms 40 to ultimately rotate counter-clockwise backpast angular limit 72. When the tension roller 38 is located betweenangular limits 70 and 72, the motor control 88 does not cause any changein the driven velocity of stretch wrapping material passing through theprestretcher. The stretch wrapping material accumulated between theroller 32 of the prestretcher, tension roller 38 and the idler roller 58supplies the demand for additional stretch wrapping material caused bythe wrapping of a corner of a load. Additionally, the tension roller 38biased by air cylinder 52 takes up surplus stretch wrapping materialduring the wrapping of a load when the demand for stretch wrappingmaterial is decreasing. The closure of the contact of microswitch 86only occurs manually during threading of the film when the dancer armsare pushed by an operator against the aforementioned spring bias whichopposes its closure. The motor control 88 includes a motor operatedpotentiometer (MOP) or equivalent electronic potentiometer 92 which maybe a model SS MOP-1 manufactured by Precision D Series Inc., 63 NicholasRoad of Framingham, Mass. and a regenerative DC motor controller 94which may be a model RG 8 manufactured by Southcon of 3608 RozzellsFerry Road, Charlotte, N.C. The output of the regenerative DC motorcontroller 94 is applied to motor 96 which is applied to drive 20 ofFIG. 1 to vary its speed. The function of the regenerative DC motorcontroller 94 is to maintain the output shaft speed of the motor 94constant independent of torque. The DC motor controlled by aregenerative controller functions as a brake to the load when the motoris being driven by the load at a speed higher than the rated speed ofthe controller for driving a load. The function of the motor operatedpotentiometer 92 or the equivalent is, upon the closure of the contactsof microswitches 80 and 84, to respectively increase and decrease themotor drive velocity for the rollers 18 and 32. The output signal whichis applied from the motor operated potentiometer to the regenerative DCmotor controller 94 is maintained at a constant potential upon thesubsequent opening of the contacts of microswitchs 80 and 84, whichpotential is maintained equal to the potential at the instance ofopening of the contacts.

FIG. 10 illustrates schematically a second form of motor control for therollers 18 and 32 which is proportionate to the angular position of thedancer arms 40 throughout almost the entire rotation of the dancer arms40. In this form of motor control, a potentiometer 100 is coupled to theaxis of rotation 101 of the dancer arms 40 by a slip coupling 102 and atransmission 104 which multiplies the angular rotation of the dancerarms (approximately 60°) into 300° of rotation to use the full range ofcommercially available rotary potentiometers. The slip coupling 102,which may be of any known design, allows for a limited degree of deadspace between initial movement of the axis of rotation 101 of the dancerarms 40 and the wiper of the potentiometer 100. The potentiometer 100functions as a means for detecting change in the velocity of thewrapping material on the load. The dead space tends to prevent overcompensation of the velocity of the rollers 18 and 32 which could cause"hunting" by not introducing a change in the motor control until achange in the sign (positive to negative or visa versa) of theacceleration of the film has occurred which has produced a net velocitychange of a magnitude sufficient to require correction. With referenceto FIG. 10, the motor control potentiometer circuit has potentiometer100 which is electrically coupled to a regenerative DC motor controller94 which may be identical to the regenerative DC motor controllerdescribed supra with regard to FIG. 9. The motor controller maintainsthe speed of the motor substantially constant.

FIG. 11 illustrates a second embodiment of the invention which does notprestretch the film before wrapping. The same reference numerals areused in FIGS. 10 and 11 to identify like parts. The second embodimentdiffers principally from the first embodiment in that the film ofwrapping material 16 is pulled from the film roll by the rotation of theload not illustrated which is resting on a turntable under theresistance of braking force which is applied by a film rollelectromagnetic brake 110. The preferred form of brake is the magneticparticle type. Once the film 16 leaves the film roll, it contacts idlerroller 112, and a constant tension mechanism, including elements 38, 40,50, 52 and 54, which is identical to the constant tension mechanismdescribed above with reference to the first embodiment. The film passesfrom roller 38 to idler roller 114 and to a load being wrapped which isnot illustrated. The wiper 116 of a rheostat 114 is connected to theaxis of rotation of the dancer arms 40 by a slip coupling 102 andtransmission 104 which are identical to those described with referenceto FIG. 10 supra. The rheostat functions as means for sensing changes inthe velocity of the film being wrapped on the load and produces anoutput voltage which is a linear function of the velocity of the film.The position of the wiper 116 of the rheostat 114 is used to generate asignal to control the braking force applied to the film roll by theelectromagnetic brake 110. The control circuit of FIG. 11 includes asource of alternating current potential 120. The alternating current isrectified by a full wave rectifier 122 which output is applied acrossthe terminals of master control rheostat 124. The setting of the wiper126 of the rheostat 124 determines the average braking force which isapplied by the electromagnetic brake 110. The greater the resistancesetting of the wiper 126, the greater average braking force which isapplied by the electromagnetic brake 100. The wiper 126 is coupled toone of the two terminals of rheostat 116 which is coupled to the axis ofrotation 101 of the dancer arms 40 as previously described. Theremaining terminal of rheostat 114 is coupled across one of the outputsof full wave rectifier 122 which is in common with the terminal ofrheostat 124. The wiper 116 of rheostat 114 is coupled to one terminalof the electromagnetic brake 110, the remaining terminal being coupledto the common terminal of the full wave rectifier 122 and the rheostats114 and 124. As is apparent from the discussion above with reference toFIG. 10, the angular movement of wiper 116 is responsive to the movementof the dancer 40 in the manner described with reference to slip coupling102 and transmission 104. The magnitude of the potential which isapplied from the full wave rectifier 122 to the electromagnetic brake isa linear function of the combined settings of the wiper arms 103 and112.

The operation of the electromagnetic brake control in varying the speedof rotation of the film roll is as follows. The constant tensionmechanism is adjusted to create the desired tension required forwrapping of an article, the actual wrapping tension being approximatelyone half of the constant biasing force which has been set on roller 38.After the desired tension is set and the film has been threaded aroundthe article to be wrapped, the master control rheostat 124 is set sothat the dancer arms will move in response to changes in velocity of thewrapping film for the desired tension setting. In operation, anysignificant change in the velocity of the film will produce acounteracting braking force on the electromagnetic brake 100 which tendsto maintain constant tension. If the sign of the (positive to negativeor visa versa) acceleration of the film changes, the slip coupling 102will not translate any movement of the dancer arm's axis of rotation 101to the wiper 116 of rheostat 114 until a certain magnitude of velocitychange has occurred, the dead space in the slip coupling tending toreduce overcorrection in braking force which could result in hunting.

While the invention has been described in terms of its preferredembodiments, it should be understood that numerous modifications may bemade to the invention without departing from its scope as defined in theappended claims.

What is claimed is:
 1. A constant tension stretch wrapping machine forwrapping a load comprising:a dispensing means for dispensing stretchablewrapping material; a turntable for supporting and rotating the load tobe wrapped; and a constant tension maintaining means disposed betweenthe dispensing means and the turntable for applying stretchable wrappingmaterial to the load with constant tension, said tension maintainingmeans including stretchable wrapping material engaging means forengaging the stretchable wrapping material between the turntable and thedispensing means, the stretchable wrapping material engaging meanshaving a rotatable member mounted for rotation through a path ofrotation around a pivot point with a roller rotatably attached theretoat a point offset from the pivot point, the roller engaging thestretchable wrapping material and defining a path of approach of thestretchable wrapping material between the dispensing means and theroller and a path of departure of the stretchable wrapping material fromthe roller, and means for applying a constant force to the rotatablemember of the stretchable wrapping material engaging means at a pointoffset from the pivot point which applies a torque to the rotatablemember which opposes a torque applied to the stretchable wrappingmaterial engaging means by tension on said stretchable wrapping materialin the paths of approach and departure, the torque applied to therotatable member by the means for applying a constant force varying as afunction of rotation of the rotatable member, being equal in magnitudeto the torque applied by the tension on the stretchable wrappingmaterial and opposing the torque applied by the tension on thestretchable wrapping material to the rotatable member whereby thetension on the stretchable wrapping material being applied to the loadis constant during the rotation of the rotatable member.
 2. A constanttension stretch wrapping machine in accordance with claim 1 wherein therotatable member of the stretchable wrapping material engaging meansincludes a first arm and a second arm which is connected to the firstarm, the first arm being pivoted about the pivot point and carrying theroller at a point offset from the pivot point, and the second arm beingpivoted about the pivot point with the means for applying a constantforce being applied to the second arm at a point offset from the pivotpoint.
 3. A constant tension stretch wrapping machine in accordance withclaim 2 wherein the first arm has a longitudinal axis which is notparallel with a longitudinal axis of the second arm.
 4. A constanttension stretch wrapping machine in accordance with claim 2 furthercomprising:means for sensing an increase in the velocity of thestretchable wrapping material being wrapped around the load; means forsensing a decrease in the velocity of the stretchable wrapping materialbeing wrapped around the load; and control means coupled to thedispensing means which is responsive to the means for sensing anincrease in the velocity of the stretchable wrapping material to causethe dispensing means to increase the velocity of stretchable wrappingmaterial being dispensed and which is responsive to the means forsensing a decrease in the velocity of the stretchable wrapping to causethe dispensing means to decrease the velocity of stretchable wrappingmaterial being dispensed.
 5. A constant tension stretch wrapping machinein accordance with claim 2 further comprising:means for sensing when therotatable member moves past a first angular limit at a boundary of afirst control zone into a second control zone within the path ofrotation that is caused by an increase in the velocity of thestretchable wrapping material being wrapped around the load; means forsensing when the rotatable member moves past a second angular limit at aboundary of the first control zone into a third control zone within thepath of rotation that is caused by a decrease in the velocity of thestretchable wrapping material being wrapped around the load; and controlmeans coupled to the dispensing means for causing the dispensing meansto increase the velocity of the stretchable wrapping material fed by thedispensing means in response to the means for sensing when the first armmoves past the first angular limit into the second control zone, forcausing the dispensing means to decrease the velocity of the stretchablewrapping material fed by the dispensing means in response to the meansfor sensing when the first arm moves past the second angular limit intothe third control zone and for causing the dispensing means to maintaina substantially constant velocity of the stretchable wrapping materialbeing fed by the dispensing means as long as the first arm is within thefirst zone.
 6. A constant tension stretch wrapping machine in accordancewith claim 5 wherein the dispensing means feeds the stretchable wrappingmaterial with an increasing velocity as long as the first arm is withinthe second control zone and feeds the stretchable wrapping material witha decreasing velocity as long as the first arm is within the thirdcontrol zone.
 7. A constant tension stretch wrapping machine inaccordance with claim 5 wherein the dispensing means includes means forprestretching the stretchable wrapping material.
 8. A constant tensionstretch wrapping machine in accordance with claim 7 wherein the meansfor prestretching includes a motor driven by a regenerative controllerwith the motor causing the stretchable wrapping material to be fed bythe dispensing means.
 9. A constant tension stretch wrapping machine inaccordance with claim 4 wherein the dispensing means includes means forprestretching the stretchable wrapping material.
 10. A constant tensionstretch wrapping machine in accordance with claim 9 wherein the meansfor prestretching includes a motor driven by a regenerative controllerwith the motor causing the stretchable wrapping material to be fed bythe dispensing means.
 11. A constant tension stretch wrapping machine inaccordance with claim 1 further comprising another roller rotatablymounted at a point between the first roller and the turntable whichengages the stretchable wrapping material during wrapping of the loadand which defines the path of departure of the stretchable wrappingmaterial between the first roller and the second roller.
 12. A stretchwrapping machine for wrapping a load comprising:a dispensing means fordispensing stretchable wrapping material; a turntable for supporting androtating the load to be wrapped; a tension maintaining means disposedbetween the dispensing means and the turntable for applying stretchablewrapping material to the load with substantially constant tension, saidtension maintaining means including an accumulator for receivingstretchable wrapping material from the dispensing means and forproviding the stretchable wrapping material to the turntable, whichmoves in response to variation of the velocity of the stretchablewrapping material being delivered to the turntable, the accumulatorbeing pivotable about a pivot point with a roller rotatably attachedthereto at a point offset from the pivot point which engages thestretchable wrapping material engaging the roller applying a torque tothe pivot point which varies as a function of rotation of theaccumulator; and means for applying a constant force to the accumulatorat a point offset from the pivot point which applies a torque to theaccumulator which varies as a function of rotation of the accumulatorand is equal to and opposes the torque applied to the accumulator by thetension on the stretchable wrapping material engaging the roller.
 13. Aconstant tension stretch wrapping machine for wrapping a load inaccordance with claim 12 wherein the dispensing means includes a meansfor prestretching.
 14. A stretch wrapping machine for wrapping a loadcomprising:a dispensing means for dispensing stretchable wrappingmaterial; a turntable for supporting and rotating the load to bewrapped; and means disposed between the dispensing means and theturntable for applying stretchable wrapping material to the load, saidmeans for applying including stretchable wrapping material engagingmeans for engaging the stretchable wrapping material between theturntable and the dispensing means, the stretchable wrapping materialengaging means having a rotatable member mounted for rotation through apath of rotation around a pivot point with a roller rotatably attachedthereto at a point offset from the pivot point, the roller engaging thestretchable wrapping material defining a path of approach of thestretchable wrapping material between the dispensing means and theroller and a path of departure of the stretchable wrapping material fromthe roller; the rotatable member and the path of approach of thestretchable wrapping material defining a first angle and the rotatablemember and the path of departure of the stretchable wrapping materialdefining a second angle, the first and second angles being substantiallyequal within the path of rotation and remaining substantially equalwhile varying through the path of rotation, and means for applying aforce to the rotatable member of the stretchable wrapping materialengaging means at a point offset from the pivot point which applies atorque to the rotatable member which opposes a torque applied to thestretchable wrapping material engaging means by tension on saidstretchable wrapping material in the paths of approach and departure.15. A stretch wrapping machine in accordance with claim 14 wherein:therotatable member of the stretchable wrapping material engaging means andthe means for applying a force define a third angle which issubstantially equal to the first and second angles within the path ofrotation.
 16. A stretch wrapping machine in accordance with claim 16wherein:the rotatable member of the stretchable wrapping materialengaging means includes a first arm and a second arm which is connectedto the first arm, the first arm being pivoted about the pivot point andcarrying the first roller at a point offset from the pivot point, andthe second arm being pivoted about the pivot point with the means forapplying a force being applied to the second arm at a point offset fromthe pivot point.
 17. A stretch wrapping machine in accordance with claim16 wherein:the first arm has a longitudinal axis which is not parallelwith a longitudinal axis of the second arm.